1. Field of the invention
The present invention relates to a flow control apparatus used in a hydraulic system.
2. Description of the Related Art
Recently, a four-wheel steering system in a vehicle is known as a hydraulic system. This system comprises a power steering device for steering front wheels by generating an oil pressure in accordance with turning of a steering wheel, a hydraulic actuator for steering rear wheels, and an oil pump arranged to be driven by, e.g., a rear-wheel differential mechanism and adapted to generate an oil pressure in accordance with a vehicle speed. A control valve is arranged in a hydraulic circuit connecting the oil pump to the hydraulic actuator. The control valve is designed so as to switch the hydraulic circuit in accordance with the oil pressure generated by the power steering device.
According to the four-wheel steering system with the above-described arrangement, the rear wheels can be steered in the same direction as the front wheels by the oil pressure generated by the oil pump while the vehicle is moving at high speed. As a result, traveling stability of the vehicle during a high-speed traveling mode can be improved. However, in a four-wheel steering system with such an arrangement, the amount of oil delivered from the oil pump is increased with an increase in vehicle speed. For this reason, the generated oil pressure of the oil pump is excessively increased within a vehicle speed range exceeding a certain speed level, thereby steering the rear wheels at an excessive steering angle and degrading durability of the hydraulic circuit. A flow control valve is easily used as means for solving such problems. The flow control valve serves to return part of working oil to the suction side of the oil pump by opening a return path when the amount of oil delivered from the oil pump exceeds a predetermined value.
The viscosity of a working oil flowing through hydraulic circuits in hydraulic systems varies depending on the oil temperature, although this problem is not limited only to such a hydraulic four-wheel steering system as described above. When the oil temperature is low, the viscosity of the working oil is high, whereas when it is high, the viscosity of the working oil becomes low. For this reason, in the above-described arrangement, even if the vehicle speed is the same (the delivery amount of the working oil from the oil pump is the same), a generated oil pressure is much higher when the oil temperature is low than when the oil temperature is high. That is, the steering angle of the rear wheels, which is determined in accordance with the oil pressure applied to the hydraulic actuator, is changed by the oil temperature, and hence steering characteristic of the rear wheels is changed by the temperature of the working oil.